Thin film, ferrimagnetic materials such as rare earth-transition metal amorphous alloys of terbium iron cobalt (TbFeCo), gadolinium terbium cobalt (GdTbCo) and gadolinium terbium iron cobalt (GdTbFeCo) have been known as high-density, magneto-optic recording media. Magnetic domains on the order of one micrometer in size can be recorded in the magneto-optic film. These ferrimagnetic materials have high coercivity at room temperature and low coercivity at high temperatures. The recording medium, preferably in a coated disk form, can be magnetized in a particular direction perpendicular to the surface by heating the disk in the presence of an external magnetic field, and thin permitting the disk to cool or by applying a saturating magnetic field. Data can thereafter be stored on the disk by heating a small spot (preferably by laser energy) in the presence of an external magnetic field of the desired magnetic polarity. The heated area is magnetized in the direction of the external magnetic field when the area cools and returns to the high coercivity state at room temperature. Data on the disk is "read" by noting the effect on polarized light reflected off the disk surface. Such systems operating with external magnetic bias operate by either heating above the compensation temperature of the medium or by heating above the Curie temperature of the medium. Systems using a reversible external magnetic field have the advantage of directly controlling the recorded magnetic state according to the polarity of the applied external field, but tend to be slow in operation.
Curie point systems operating without external magnetic bias are also known. See Japanese patent application No. 59/1984-113506 "Method and Apparatus for Opto-Magnetic Recording, Reading and Erasure", filed Dec. 21, 1982, published June 30, 1984, and identifying M. Okada et al as inventors. The magneto-optic medium in this system has a relatively low Curie point, in the range of 80.degree. C. to 180.degree. C. Where an area is heated above the Curie temperature, the heated area loses its magnetization and, upon cooling, forms a stable domain of reverse magnetic polarity at approximately one-half the radius of the area heated above the Curie point. To erase a previously recorded domain, the previously recorded domain area is heated above the Curie point and the magnetization of the heated area disappears. If the erase pulse is small and just sufficient to heat the area of the prior domain above the Curie point, the domain of reverse magnetic polarity that tends to form upon cooling is unstable and therefore collapses during cooling.
With the Curie point system described by Okada et al, recording and erasing are achieved by a single-beam, direct over-write operation. The laser pulse used to record a spot domain is greater than the laser pulse required to erase that domain. Therefore, a large record pulse creates a domain indicating a "1" state regardless of prior magnetic history and, likewise, a smaller erase pulse results in the absence of a domain indicating a "0" state regardless of prior magnetic history. In both cases, the area heated above the Curie point loses its magnetism thereby wiping out the prior magnetic history. The existence of a domain, or lack thereof, when the area cools depends on the size of the area heated above the Curie point.